Conventionally, a Ziegler-Natta catalyst system consisting of a titanium or vanadium compound as main catalyst component and an alkylaluminum compound as cocatalyst component has been used for the preparation of ethylene homopolymers or copolymers thereof with α-olefin. Although the Ziegler-Natta catalyst system is highly active in the polymerization of ethylene, it has heterogeneous catalytic active sites, leading to a polymer having a broad molecular weight distribution and, and in particular, a copolymer of ethylene and α-olefin having a non-uniform compositional distribution.
Recently, the so-called metallocene catalyst system has been developed, which consists of a metallocene compound of a group IV transition metal such as titanium, zirconium, hafnium, etc. and a methylaluminoxane cocatalyst. Since the metallocene catalyst system is a homogeneous catalyst having homogeneous catalytic active sites, it can provide a narrower molecular weight distribution than the Ziegler-Natta catalyst system and can be used to prepare polyethylene having uniform compositional distribution. For example, European Patent Publication Nos. 320,762 and 372,632 and Japanese Patent Laid-open Nos. Sho 63-092621, Hei 02-084405 and Hei 03-002347 disclose metallocene compounds such as Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, ethylene(IndH4)2ZrCl2, activated with methylaluminoxane as a cocatalyst to polymerize ethylene at high catalytic activity, thereby making it possible to produce polyethylene having a molecular weight distribution (Mw/Mn) of 1.5-2.0. However, it is difficult to obtain polymers having high molecular weights using the catalyst system. Further, in the case where such a catalyst system is applied to solution polymerization at high temperatures of 140° C. or above, polymerization activity is drastically decreased, and β-dehydrogenation predominates, and thus the catalyst system is known to be unsuitable for the production of high molecular weight polymers having a weight average molecular weight (Mw) of 100,000 or more.
In addition, the so-called non-metallocene catalyst having a geometrically constrained structure (also known as single active site catalyst) has been presented, in which a transition metal is linked to form a ring, as a catalyst for the preparation of high molecular weight ethylene homopolymers or copolymers of ethylene and α-olefin by solution polymerization with high catalytic activity. European Patent Publication Nos. 0416815 and 0420436 disclose a catalyst having a geometrically constrained structure in which an amide group is linked to a single cyclopentadiene ligand to form the shape of a ring. European Patent Publication No. 0842939 discloses a catalyst in which a phenolic ligand as an electron donating compound is linked to a cyclopentadiene ligand to form a ring. However, such geometrically constrained catalysts are commercially inapplicable because the yield of ring formation between the ligand and the transition metal compound during the catalyst synthesis is very low.
Further, non-metallocene catalysts without having a geometrically constrained structure but applicable in high temperature solution conditions are disclosed in U.S. Pat. No. 6,329,478 and Korean Patent Publication No. 2001-0074722. In these patents, single active site catalysts having at least one phosphinimine compound as ligand are used to provide high ethylene transition ratio in copolymerization of ethylene and α-olefin under high temperature solution polymerization condition of 140° C. or higher. However, they required the use of a phosphine compound for the synthesis of the phosphinimine ligand. Because the compound is harmful to the environment and humans, the catalysts are inapplicable to the production of general-use olefin polymers. U.S. Pat. No. 5,079,205 discloses a catalyst having a bis(phenoxide) ligand, but the catalytic activity is too low to be commercially applicable.
In addition, a non-metallocene catalyst having a phenolic ligand and its application for polymerization are disclosed in Organometallics 1998, 17, 2152 (Nomura et al.). However, the alkyl substituent is restricted to an isopropyl group.